1. Field of the Invention
The present invention relates to an organic electroluminescence device having an organic film layer.
2. Description of the Related Art
As a film type electroluminescence (hereinafter, the term “electroluminescence” may occasionally be referred to as “EL”) device, devices in which ZnS, CaS, SrS and the like, which are inorganic materials and specifically II-VI group compound semiconductors, are doped with Mn of an emission center or a rare earth element (Eu, Ce, Tb, Sm etc.), are generally used. However, EL devices made from the aforementioned inorganic material have a problem that 1) alternating current driving is necessary (generally, 50 to 1000 Hz), 2) a driving voltage is high (generally, around 200 V), 3) full colorization is difficult and, in particular, blue emitting is problematic, and 4) the cost of a peripheral driving circuit is high.
On the contrary, in organic EL devices using an organic compound as a light-emitting layer and the like, a driving voltage can be considerably reduced, the size of the device can be easily made compact, consumed power is small, plane emitting is possible, and three primary colors emission is easy. Therefore, organic EL devices are being keenly studied and exploited as a next generation light-emitting device. In particular, the light-emission efficiency of the recent organic EL devices has significantly been enhanced, as compared with that of the conventional EL devices using a single crystal of anthracene and the like, as a result of the development of organic EL devices having a laminated structure in which the effect of injecting carriers from an electrode is enhanced by optimization of an electrode species, and a hole transporting layer consisting of aromatic diamine and a light-emitting layer consisting of an aluminum complex of 8-hydroxyquinoline are provided (refer to, for example, non-patent reference 1 described below).
Further, in order to prolong the device life, research on various materials was made. For example, as a material for a hole injecting layer, porphyrin derivatives and phthalocyanine compounds (for example, see patent reference 1 below), star burst type aromatic triamine (for example, see patent reference 2 below), hydrazone compounds (see, patent reference 3 below), alkoxy-substituted aromatic diaimne derivatives (for example, see patent reference 4 below), p-(9-anthryl)-N,N-di-p-tolylaniline and the like have been exploited, which, to some extent, exhibit properties applicable to actual use. Further, besides electroluminescence devices using the aforementioned low-molecular materials, exploitation of EL devices using a polymer material such as poly(p-phenylenevinylene), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene], poly(3-alkylthiophene) and the like as a material for a light-emitting layer, and exploitation of devices in which a low-molecular light-emitting material and an electron transferring material are mixed into a polymer such as polyvinylcarbazole and the like, have been made.
As the construction of an organic EL device, the structure including anode/organic light-emitting layer/cathode in this order is the basic construction. A construction in which a hole injecting-transporting layer and an electron injecting layer are further provided in an appropriate manner, to the aforementioned basic construction, is also known. Specifically, for example, a construction including anode/hole injecting-transporting layer/organic light-emitting layer/cathode in this order and a construction including anode/hole injecting-transporting layer/organic light-emitting layer/electron injecting layer/cathode in this order are known.
The greatest task in exploitation of the aforementioned organic EL device is prolongation of life at driving. Examples of the problematic phenomenon at driving include reduction in the luminance, rise in voltage at constant current driving, occurrence of a non-emitting part (dark spot) and the like. Although there are some causes for these occurrences, deterioration of a film shape in an organic layer is the main cause. This deterioration of a film shape is thought to be derived from crystallization (or aggregation) of an organic amorphous film due to heat generation at device driving and the like.
By the way, as a material for an organic EL device, an organic low-molecular compound and an oligomer are conventionally used by vacuum deposition in many cases. However, the organic low-molecular compound and the oligomer have the low heat resistance and, therefore, improvement of them have been sought. For the purpose of preventing such defects due to crystallization and the like from occurring, polymerizing the aforementioned organic low-molecular compound has been attempted, to improve the thermal stability. Specifically, an attempt to improve the heat resistance has been made by using a polymer having a particular structure.
Further, study of impartment of the higher heat resistance by three-dimensionally cross-linking the aforementioned polymer or the like was also studied, and there is disclosed that a compound having an alkoxysilyl group is heated to cure by a sol-gel method (see patent references 5 and 6, and non-patent references 2 to 4 below).
[Patent Reference 1]
Japanese Patent Application Laid-Open (JP-A) No. 63-295695
[Patent Reference 2]
JP-A No. 4-308688
[Patent Reference 3]
JP-A No. 4-320483
[Patent Reference 4]
JP-A No. 4-220995
[Patent Reference 5]
JP-A No. 9-279135
[Patent Reference 6]
JP-A No. 2000-80167
[Non-Patent Reference 1]
Appl. Phys. Lett., 51,913, (1987)
[Non-Patent Reference 2]
56th Applied physics Annual Meeting Pre-Manuscript, Vol. 3, p.1031 (1995)
[Non-Patent Reference 3]
Adv. Mater., 11, No.2 (1999)
[Non-Patent Reference 4]
Adv. Mater., 11, No.9 (1999)
A film cured by heating by the aforementioned sol-gel method is not only excellent in the heat resistance but also excellent in the adherability with an inorganic electrically-conducting film such as ITO to which the prior art organic materials have the poor adherability, and is very effective as a hole or electron injecting layer. In addition, since the above film can be coated with a solvent which does not dissolve a polymer, such as alcohol and the like, it has the characteristics that lamination and coating on the surface of a small molecule or a polymer is also easy. However, on the other hand, it is difficult to completely remove a water molecule produced as a result of condensation from a film, and there is a problem that corrosion of an electrode by a moisture in a film easily occurs in a long term use.